At present, as is known, the wine industry makes widespread use of microfiltration plants, or submicronic filtration (ultrafiltration or nanofiltration) plants, which use semipermeable membranes generally formed by thin polymer films in order to separate some components from others.
The passage of the wine to be filtered through the membrane is usually obtained by means of mechanical overpressure. These membrane-type filtration plants have major operational limitations in that they do not allow the separation of all the undesirable components, for example because of their dimensions which in some cases are excessively small, their polarity or their surface tension. These plants, therefore, are suitable only for implementing given filtration processes of the type for example described in the patents IT 1,249,187 and IT 1,242,866.
Moreover, numerous plants based on the principle of (direct or reverse osmosis) are known, said plants being able to achieve, by means of a semipermeable membrane, purification of solutions, separating a permeated fraction from a retained fraction. Reverse osmosis in particular is a known dual-action process which allows, on the one hand, the purification of solutions, eliminating undesirable substances (for example salts) and, on the other hand, the concentration of solutions, extracting water or other solvent from the initial product.
In the oenology sector, reverse osmosis is currently used in methods for concentrating wine or fruit juices with a low sugar content, as described in the U.S. Pat. No. 3,979,521, or in order to extract volatile esters with a low molecular weight (such as, for example, methyl-anthranilate) from wine or must, as described in the U.S. Pat. No. 4,401,678, or in order to remove water or a more or less important part of alcohol from wine as described in the patents U.S. Pat. No. 4,999,209, U.S. Pat. No. 4,888,189, U.S. Pat. No. 4,681,767, FR 2,653,443, GB 2,133,418, IT 1,242,865, WO/03902 and WO/08783.
A method and an apparatus for the treatment of fluids and in particular wine, able to separate a retained fraction from a permeated fraction in a reverse osmosis unit, is also known from the U.S. Pat. No. 5,480,665. The latter receives inside it the undesirable substances such as, for example, the volatile acidity components (ethyl acetate and acetic acid) which are then removed by means of a subsequent treatment. For this purpose, the permeated fraction is passed through an adsorbent column able to remove the acetic acid by means of anion-exchange resins and the ethyl acetate by means of hydrolysis.
With this treatment it is therefore possible to purify, of the undesirable substances, the permeated fraction which is then reintroduced into the wine, obtaining a final product which is substantially devoid of volatile acidity. Advantageously, this method envisages also treating the permeated fraction with a low-temperature distillation process in order to remove CO2, H2S, acetaldehyde or other volatile components.
Methods for treating wine in order to lower the total acidity by means of ion-exchange resin columns are per se already known and for example described and illustrated in the U.S. Pat. No. 4,205,092.
The processes for separation of undesirable substances briefly described hitherto have in practice proved to be unsatisfactory in terms of performance. In other words, with these processes and associated plants it is not possible to select adequately only the undesirable substances, and therefore inevitably also substances—which instead make a positive contribution to the quality of the wine—are removed together therewith.